This invention relates to a rack and pinion type focussing mechanism having a converting mechanism of lateral and longitudinal camera positions in a photographic lens mount. Lens mounts for lenses having long focal lengths are large in total length, outer diameter and weight. Most lens mounts, therefore, have tripod mounting portions used to secure the lens when used in photography. It is often required to take a telephoto or wide angle picture on film corresponding to an object to be photographed. Therefore, for such photography it is required to take the picture with either a lateral or longitudinal camera position to properly frame the subject. If the picture is taken while the camera is siezed directly by the operator's hands, any camera position can obviously be freely selected.
In a lens having such weight of lens mount that suitable photographing can be accomplished when a tripod is secured to the camera body side, it is also possible easily to convert between the lateral and longitudinal camera position in the tripod head therefor. However, in a lens mount for a lens having a long focal length, wherein a tripod mounting portion is not provided on the camera side but on the lens mount side, a lateral-longitudinal camera position converting mechanism must be provided on the lens mount side.
In a manual stopping-down lens mount, it is possible to convert the camera position irrespective of a focussing mechanism by providing the lateral-longitudinal camera position converting mechanism in the vicinity of the camera mounting portion since no cooperating member between the mount and the camera is required for stopping-down. On the other hand, in a lens mount having an automatic stopping-down wherein a cooprating member between the lens mount and camera sides is required, it is required to carry out the 90.degree. lateral-longitudinal camera position conversion with maintaining the relative position of the cooperating member constant. Accordingly, there is, generally provided such systems, a tripod mounting seat on the lens mount side that is rotatable with respect to the optical axis in order to achieve the 90.degree. camera position conversion of the overall lens mount including a camera body with respect to the tripod mounting seat. However, in this method, since the overall lens mount is rotated with respect to the tripod mounting seat to thereby achieve the 90.degree. camera position conversion, the focussing mechanism is also rotated by 90.degree. therewith.
The following defects of this technique are inevitable. First, in view of the mode of operation of the focussing mechanism, a rack and pinion type is far superior to a helicoid type for the focussing mechanism which is heavy and has a large operational portion diameter, wherein the amount of the lens advancing during focussing is large. However, the helicoid type is generally used in the camera position converting system in which the lens mount is rotated with respect to the tripod mounting seat. This is the reason why in the rack and pinion type focussing mechanism, though focussing operational member having a good operationability is established for a lateral camera position, when the 90.degree. camera position conversion is carried out for the longitudinal position, the focussing operational member is rotated to a position where the ability to effectively operate the mechanism is deteriorated.
Furthermore, in order to prevent a blurred picture from being generated, the tripod mounting portion of such a lens mount must be provided as far as possible from the center of gravity of the overall lens mount including a camera body. This will provide for the most stable camera platform. In view of photographic operation when using a tripod, it is generally desired to provide the tripod mounting seat near the camera. It is essential for good operation to lighten the weight of the camera-lens portion which is moved by focussing. It is therefore preferable to provide the focussing operational member at a closer position to the camera body than the tripod mounting seat.
When the outer projected portions such as the focussing operational member of the rack and pinion type are rotated by 90.degree., the outer projected portions are brought into contact with the tripod head. For this reason, it is necessary to minimize the projected portions of both the tripod mounting seat and the rack and pinion type operational member to prevent the projected portions from abutting with the tripod or to widen the spaced relation to prevent the contact therebetween. The former case sacrifices ease of operation and the latter case also sacrifices stability of the system, tending to blur image photographed.
On the other hand, in a helicoid type focussing mechanism, the operational portion diameter is large and the rotational angle is large. Therefore, as mentioned the helicoid type is inferior to the rack and pinion type. However, since the overall outer circumference of the body of revolution with respect to the optical axis can be the focussing operational member itself, the operation of the mechanism is not deteriorated even after the 90.degree. camera position conversion. Since there are no projected portions on the outer periphery thereof, a substantial degree of freedom exists for the establishment of a positioning relation to the tripod mounting seat. A conventional focussing mechanism of the lens mount having an automatic stopping-down mechanism and the lateral and longitudinal camera position converting mechanism is generally a helicoid type.